Restframe Optical Spectroscopic Classifications for Submillimeter Galaxies

Abstract

We report the results of a systematic near-infrared spectroscopic survey using the Subaru, VLT and Keck Telescopes of a sample of high redshift Ultra-luminous Infrared Galaxies (ULIRGs) mainly composed of submillimeter-selected galaxies. Our observations span the restframe optical range containing nebular emission lines such as H_beta, [OIII], and [OII], which are essential for making robust diagnostics of the physical properties of these ULIRGs. Using the H_alpha/H_beta emission line ratios, we derive internal extinction estimates for these galaxies similar to those of local ULIRGs:A_v~2.9+-0.5. Correcting the H_alpha estimates of the star formation rate for dust extinction using the Balmer decrement, results in rates which are consistent with those estimated from the far-infrared luminosity. The majority (>60%) of our sample show spectral features characteristic of AGN, with ~65% exhibiting broad Balmer emission lines. A proportion of these sources show relatively low [OIII]/H_beta line ratios, which are similar to those of Narrow Line Seyfert 1 galaxies suggesting small mass black holes which are rapidly growing. In the subsample of our survey with both [OIII] and hard X-ray coverage, at least ~60% show an excess of [OIII] emission, by a factor of 5-10 times, relative to the hard X-ray luminosity compared to the correlation between these two properties seen in Seyferts and QSOs locally. From our spectral diagnostics, we propose that the strong [OIII] emission in these galaxies arises from shocks in dense gaseous regions in this vigorously star-forming population.